Current research concerns: (1) Increased UV(254 nm)-resistance of repair-deficient E. coli cells, resulting from infection with heavily UV-irradiated phage T4, which reflects cell recovery due to repair processes made possible by contribution from the phage genome; (2) Host-cell reactivation of UV-irradiated phage T1 by bacterial excision repair, which is studied by applying repair inhibitors (caffeine, acriflavine), liquid-holding of the complexes, or by using pre-irradiated host cells. The occurrence of 2 modes of excision repair operating on T1 DNA, which is indicated from this work, is characterized; (3) Reasons for the lack of host-cell reactivation of UV-irradiated phage T5. Experimental work envisaged includes several aspects of photoenzymatic repair, notably the activation of photoreactivating enzyme in vitro by light (up to 650 nm) prior to its use in the repair reaction, and the occurrence of a 2-photon effect required for maximal effectiveness of photoreactivating light. BIBLIOGRAPHIC REFERENCES: W. Harm: Dark recovery of UV-irradiated phage T1. II. Interpetation of the survival kinetics obtained under conditions of host-cell reactivation. Mutation Res. 25, 3-14 (1974). W. Harm: Dark recovery of UV-irradiated phage T1. III. Evidence for two modes of host-cell repair from liquid-holding experiments. Mutation Res. 25, 15-24 (1974).